docs: add test future-proofing and large-scale refactoring lessons

Document key lessons from chime.el timestamp refactoring project:

## New Sections Added

**Test Future-Proofing & Time-Based Testing**
- Dynamic timestamp generation patterns and benefits
- Never hardcode dates in tests - use relative time helpers
- Mock time via function substitution (with-test-time pattern)
- Code examples showing before/after patterns

**Large-Scale Test Refactoring Strategy**
- Strategic planning: tackle biggest challenges first
- Execution approach: maintain 100% pass rate throughout
- Project management: track progress visibly, celebrate milestones
- Know when you're done: not all files need changes

**Real-World Example**
- chime.el project: 23 files, 339 tests
- 16 files refactored (251 tests), 7 files skipped (88 tests)
- 100% pass rate maintained across all refactoring
- Result: future-proof test suite that never expires

## Key Insights

- "Tackle biggest challenge first" eliminates intimidation
- Work in batches but commit individually for clean history
- Don't let perfectionism create unnecessary work
- Strategic approach builds momentum and confidence

Added "Hardcoded dates in tests" to Red Flags section.

These lessons capture the methodology that successfully completed
the hardest refactoring task in the project.

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